A continuing program is being conducted for the routine analyses of feed samples for chlorinated hydrocarbons, polychlorinated biphenyls, organophosphates, lead, arsenic, cadmium, mercury, nitrate and aflatoxins. A sample collected from each batch of animal feed manufactured under NIH contracts is submitted to an independent laboratory for assay. The objective of this program is to document the concentrations of these contaminants in the natural ingredient diets purchased for use at NIH and to accumulate data that could be used as a basis to establish maximum permissible concentrations in laboratory diets and to ascertain which of these are a real problem.